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Guide to Cal-Atlas

February 18, 2014

Please E-mail comments, corrections and additions to the webmaster at pje@efgh.com.

Topographic maps of California were downloaded from the CA-SIL (California Spatial Information Library). I've been chasing this library around the Internet for the last few years. It was formerly located on the UC Davis website, but now it resides on a website called Cal-Atlas on the State of California website.

The page containing topographic maps can be rather hard to find. A URL that works now (February 18, 2014) is http://atlas.ca.gov/download.html#casil/imageryBaseMapsLandCover/baseMaps/drg. If this link fails, try Googling some of the words in it.

There are several kinds of topographic maps. The ones in this website are from the "7.5 minutes series albers nad83 untrimmed" group.

Finding a particular quadrangle requires some knowledge of the naming conventions.

Quadrangles are generally aligned approximately with standard parallels of latitude and meridians of longitude. Since there are 60 minutes in one degree of latitude or longitude, there are eight 7.5-minute quadrangles in a degree of latitude or longitude. An area extending one degree from north to south and one degree from east to west contains 64 quadrangles.

On the Cal Atlas website, the map for a particular quadrangle resides in a TIF (Tagged Image Format) file with specifications of the form oaabbbcc.tif, where

o lowercase letter 'o'
aatwo-digit latitude in degrees north of the equator (ignoring any fractional degrees, minutes or seconds)
bbbthree-digit longitude in degrees west of the prime meridian (ignoring any fractional degrees, minutes or seconds)
cca two-character code that indicates the position of the quadrangle in the one-degree rectangle specified by aa and bbb, as shown below.
                                        -- W bbb° 30' 0"
                                       |
                 W bbb° 37' 30" --     |     -- W bbb° 22' 30"
                                  |    |    |
             W bbb° 45' 0" --     |    |    |     -- W bbb° 15' 0"
                             |    |    |    |    |
       W bbb° 52' 30" --     |    |    |    |    |     -- W bbb° 7' 30"
                        |    |    |    |    |    |    |
W (bbb+1)° 0' 0" --     |    |    |    |    |    |    |     -- W bbb° 0' 0"
                   |    |    |    |    |    |    |    |    |
                   ----------------------------------------- N (aa+1)° 0' 0"
                   | h8 | h7 | h6 | h5 | h4 | h3 | h2 | h1 |
                   ----------------------------------------- N aa° 52' 30"
                   | g8 | g7 | g6 | g5 | g4 | g3 | g2 | g1 |
                   ----------------------------------------- N aa° 45' 0"
                   | f8 | f7 | f6 | f5 | f4 | f3 | f2 | f1 |                    N
                   ----------------------------------------- N aa° 37' 30"      |
                   | e8 | e7 | e6 | e5 | e4 | e3 | e2 | e1 |                 W--|--E
                   ----------------------------------------- N aa° 30' 0"       |
                   | d8 | d7 | d6 | d5 | d4 | d3 | d2 | d1 |                    S
                   ----------------------------------------- N aa° 22' 30"
                   | c8 | c7 | c6 | c5 | c4 | c3 | c2 | c1 |
                   ----------------------------------------- N aa° 15' 0"
                   | b8 | b7 | b6 | b5 | b4 | b3 | b2 | b1 |
                   ----------------------------------------- N aa° 7' 30"
                   | a8 | a7 | a6 | a5 | a4 | a3 | a2 | a1 |
                   ----------------------------------------- N aa° 0' 0"

For example, the POWAY quadrangle shows the area between W 32° 52' 30" and W 33° 0' 0" in latitude and between N 117° 0' 0" and N 117° 7' 30" in longitude. Therefore, it resides in the file o32117h1.tif.

A code of this kind also appears on some quadrangle maps.

The maps posted on CA-SIL have been modified slightly. In most cases, the process was as follows:

  1. Remove the map from its GZ/TAR archive (in early library versions).
  2. Straighten up the image (most were slightly tilted).
  3. Reduce the size by 50%.
  4. Crop off excess blank space at the edges.
  5. Convert the image to GIF format.

The scale of the images, after processing, is 528 pixels per mile.